Computational Study on the Mechanisms and Rate Constants for the O(P,D) + OCS Reactions.

The mechanisms and kinetics of O(P,D) + OCS(XΣ) reactions have been studied by the high-level G2M(CC2) and CCSD(T)/6-311+G(3df)//B3LYP/6-311+G(3df) methods in conjunction with the transition-state theory and variational Rice-Ramsperger-Kassel-Marcus theory calculations. The result shows that the triplet surface proceeds directly by abstraction and substitution channels to produce SO(P) + CO(XΣ) and S(P) + CO(X Σ) by passing the barriers of 7.6 and 9.1 kcal·mol at the G2M(CC2)//B3LYP/6-311+G(3df) level, respectively, while two stable intermediates, LM1 (OSCO) and LM2 (SC(O)O), are formed barrierlessly from O(D) + OCS(XΣ) in the singlet surface, which lie at -40.5 and -50.1 kcal·mol relative to O(P) + OCS(XΣ) reactants and decompose to CO(XΣ) + SO(aΔ) and S(D) + CO(XΣ). LM1 and LM2 may also be produced by singlet-triplet surface crossings via MSX1 and MSX2; the predicted total rate constant for the O(P) + OCS(XΣ) reaction including the crossings, 9.2 × 10 exp(-5.18 kcal·mol/) cm molecule s, is in good agreement with available experimental data. The branching ratio of the CO product channel, 0.22-0.32, between 1200 and 1600 K, is also in excellent agreement with the value of 0.2-0.3 measured by Isshiki et al. ( 2464).